Hardness suppression in urea solutions

ABSTRACT

The tendency of aqueous solutions of urea and other NH-containing compositions to force instability of hardness factors has been found to be detrimental to processes and apparatus employing them. The reliability of these processes and apparatus is improved by the inclusion of hardness-suppressing compositions, which preferably include both a water-soluble polymer and a phosphonate. In particular, agricultural and NO x  -reducing applications are improved, especially for solutions containing urea hydrolysis products and the salts of them.

RELATED APPLICATIONS

This application is a continuation-in-part of add commonly assigned U.S.Patent Application entitled "Composition for Introduction Into a HighTemperature Environment" having Ser. No. 07/576,424, filed Nov. 27,1990, now abandoned, which in turn is a continuation-in-part of U.S.patent application Ser. No. 07/187,943, filed on Apr. 29, 1988, (nowabandoned) in the names of Epperly, Sprague, and von Harpe, thedisclosures of which are incorporated herein by reference.

TECHNICAL FIELD

The invention relates to improving the reliability of solutions of urea,its hydrolysis products, and related amidozine (NH₂ ) generatingmaterials; and, in particular, to the inhibition of scaling,precipitation, and other forms of solids formation and/or deposit whichare exacerbated by the presence of those compounds. Processes andcompositions are provided.

A number of commercial applications require aqueous solutions of urea tobe supplied in a reliable manner. Particularly significant uses arefound in agriculture and for reducing the emission of nitrogen oxidesfrom combustion sources.

Urea is a valuable source of nitrogen for growing crops in good yield.It can be applied as the pure compound, in chemically combined orcomplexed form, or as a hydrolysis product or mixture. When aqueoussolutions are prepared, they become more problematic than the availablemakeup water. It is now understood that this is due to the effect thatthe urea and related materials have on the hardness of the water. Itwould be desirable to reduce the problems associated with storage anduse of these aqueous liquids, especially where frequent draining,flushing and washing are inconvenient and costly in terms of both timeand labor.

Aqueous solutions of urea are also useful in reducing the environmentaldamage caused by large-scale combustion. Carbonaceous materials,including the typical hydrocarbon fuels such as coal, oil and gas, aswell as refuse, are burned in increasing amounts each year. Combustion,unfortunately, produces a variety of pollutants which must be removedfrom the effluents or adverse consequences to the environment will besuffered. To maximize the removal or reduction of pollutants, it isessential to have control systems which are reliable. Reliability ismandated by law and logic.

Among the pollutants are nitrogen oxides, referred to as a group asNO_(x). A number of strategies have been developed for reducing NO_(x)levels, prominent among which is selective non-catalytic reduction(SNCR), disclosed for example by Lyon in U.S. Pat. No. 3,900,554 and byArand et al in U.S. Pat. Nos. 4,208,386 and 4,325,924. Briefly, thesepatents disclose that ammonia (Lyon) and urea (Arand et al) can beinjected into hot combustion gases to selectively react with NO_(x) andreduce it to diatomic nitrogen and water.

The attainment of consistent, high reductions in NO_(x) is a matter ofconsiderable engineering and chemistry. These gas-phase SNCR reactionstypically involve NO_(x) levels of 100 to 1500 parts per million andeither urea or ammonia at from one to three times the amountstoichiometrically required. Thus, the reaction requires mating of thereactive materials in high dilution, and typically starts with theNO_(x) -reducing materials in aqueous droplets. The NO_(x) -reducingmaterial must be dispersed uniformly and continuously throughout the gasstream being treated to achieve contact with the NO_(x) molecules in thetemperature range effective for reaction, e.g., from 1600° to 2000° F.

Selective catalytic reduction (SCR) is similar to SNCR, but entails theuse of a catalyst and operates at lower temperatures, generally withinthe range of from 100° to 900° F. See in this regard U.S. Pat. Nos.3,032,387 and 3,599,427. The use of catalysts is effective but issensitive to particulates and increases initial and operating costs inmany situations.

Consistency in NO_(x) reduction, especially while maintaining low levelsof ammonia slip, is made even more difficult by the fact that thetemperature across any plane varies significantly at any given time andshifts with changes in rate of combustion (i.e., load) which is commonfor boilers used in power generation and other combustors. To maximizeNO_(x) reduction, the art has developed to the state where chemicals canbe injected in stages (U.S. Pat. No. 4,777,024 to Epperly et al), withvariation in location of injection and chemical formulation as isnecessary to meet the temperature and compositional variations in thegas stream being treated (U.S. Pat. No. 4,780,289 to Epperly et al). Allpiping, pumps, nozzles and associated equipment must be kept clean andclear for the objectives to be met. Frequent draining, flushing andwashing are not possible without severe consequences.

In commonly-assigned U.S. patent application Ser. No. 07/576,424, thereis disclosed a low-cost composition for reducing nitrogen oxides whichimproves delivery of active chemicals to a high temperature zone byreducing the tendency of lines and nozzles to clog or otherwise becomeobstructed. As part of that disclosure, there are identified severalsequestering agents and antiscalants to mitigate the effects of waterhardness.

As part of work more recently undertaken to find the best way to improvereliability in NO_(x) reduction systems, it was found that thenitrogen-based NO_(x) -reducing agents, which release the amidozineradical, produce a greater scaling problem than might ordinarily havebeen expected. This is especially true where used with dilution waterwhich has significant hardness such as calcium, magnesium and carbonate.It would be desirable to provide a scale inhibition system which metthese challenges reliably, regardless of the hardness level of the wateremployed to make up the aqueous solution of treatment chemicals employedfor NO_(x) reduction. It would also be desirable to provide theseimprovements for agriculture and other uses as well.

DISCLOSURE OF INVENTION

It is an object of the invention to improve the reliability of apparatuswhich handle aqueous solutions of amidozine-generating compositions.

It is an object of the invention to improve the reliability of apparatussuch as conduits, nozzles, storage vessels, pumps, and the like whichare employed to prepare, store, transport, meter, distribute, dispense,or otherwise handle aqueous solutions of urea, its precursors, itshydrolysis products, and related amidozine-generating compositions.

It is a more specific object of one aspect of the invention to improvethe reliability and reduce the maintenance of agricultural apparatusemployed to prepare, store, transport, meter, distribute, dispense, orotherwise handle aqueous solutions of urea, its precursors, itshydrolysis products, and related amidozine-generating compositions.

It is a further specific object of another aspect of the invention toimprove the reliability of apparatus such as conduits, nozzles, storagevessels, pumps, and the like which are employed to prepare, store,transport, meter, distribute, dispense, or otherwise handle aqueoussolutions of urea, its precursors, its hydrolysis products, and relatedamidozine-generating compositions employed in NO_(x) -reducinginstallations which depend on the use of these compositions as activeNO_(x) -reducing agents.

It is another object of the invention to improve the reliability ofNO_(x) -reducing installations which depend on the introduction ofaqueous solutions of NO_(x) -reducing agents into a high temperatureenvironment.

It is a further object of the invention to suppress hardness in aqueoussolutions of NH-containing agricultural compositions and NO_(x)-reducing agents during long-term storage.

It is yet another object of the invention to increase the service lifeof and/or time between overhauls for individual pieces of equipment, aswell as systems, employed to prepare, store, transport, meter,distribute, dispense, or otherwise handle aqueous solutions of urea, itsprecursors, its hydrolysis products, and related amidozine-generatingcompositions by suppressing hardness which, it has now been found, isexacerbated by these agents.

These and other objects are achieved by the present invention whichprovides improved processes and compositions.

The terms "suppress hardness" and "hardness suppressing" refer to theeffect or property of a composition to reduce the tendency of hardnessfactors in solution to form scale or precipitates. Compositions areeffective if scale or precipitation is reduced by any mechanism, butchelation and threshold inhibition are the typical mechanisms. While"scale" is considered by some as hard, adherent, heat-transfer-hinderingand nozzle-orifice-blocking solids, no differentiation is made herebetween types of scale or between scale and other forms of"precipitates". The invention in its broad aspects encompassesinhibition and prevention of scale formation and dispersion andstabilization of precipitates.

Hardness factors of particular concern include Ca²⁺, Mg²⁺, SO₄ ²⁻, Fe²⁺,Fe³⁺, HCO₃ ⁻ silica Mn²⁺, Mn⁴⁺, Cu⁺, Cu²⁺, Zn²⁺, PO₄ ³⁻ and CO₃ ²⁻, andcan include various particulates and other impurities in the water andother solution ingredients. The hardness factors and amounts presentwill vary with the objectives of specific embodiments. Throughout thisdescription, concentrations of hardness factors (e.g., "H", Ca, Mg) areexpressed as calcium carbonate.

In one broad aspect, the invention provides a process for improving thereliability, e.g., decreasing required service, of equipment employed toprepare, store, transport, meter, distribute, dispense, or otherwisehandle aqueous solutions of urea, its precursors, its hydrolysisproducts, and related amidozine-generating compositions, comprisingincorporating in the solution a hardness-suppressing compositioncomprising at least one member selected from the group consisting ofpolymers, phosphonates, chelants, phosphates, and mixtures of any two ormore of these, in an amount effective to suppress hardness.

In one specific aspect of the invention, the application of nitrogenousmaterials for fertilization is improved by: preparing an aqueoussolution comprising at least one NH-containing nitrogenous fertilizer,and a hardness-suppressing composition as defined; and, applying thesolution in a manner effective to supply nitrogen to vegetation,preferably by spraying.

In another of its more specific aspects, an improvement is provided inthe known process for reducing the concentration of nitrogen oxides in agas stream by preparing an aqueous solution of NO_(x) -reducing agent,transporting the solution to the gas stream through conduits effectivefor this purpose and introducing the solution into the gas stream at aconcentration and at a rate effective to reduce NO_(x) under theconditions prevailing in the gas stream, the improvement comprising:incorporating in the solution a hardness-suppressing composition in anamount effective to suppress hardness.

In one preferred form of the invention, the type and concentration ofthe hardness-suppressing composition are effective to protect againstcalcium scale formation at most practical calcium concentrations (e.g.,up to at least 25, and preferably above this up to about 2500 parts permillion). The pH of the aqueous solution is typically above 5, andgenerally within the range of from 7 to 11.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood and its advantages will bebetter appreciated from the following detailed description, especiallywhen read in light of the accompanying drawing wherein:

The FIGURE is a bar graph illustrating the impact of urea on hardnessinstability at several levels of total hardness from 45 to 2250 ppm.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention will be described below with reference to severalrepresentative processes and compositions to fully describe itssignificant features while avoiding unnecessary detail. It will beunderstood, however, that the invention is of broad applicability.

The invention improves the reliability of systems which employ aqueoussolutions of NH-containing compositions. For example, it enablesdecreasing the service (including simple draining, flushing and washing,as well as mechanical repair) required for equipment employed toprepare, store, transport, meter, distribute, dispense, or otherwisehandle aqueous solutions of urea, its precursors, its hydrolysisproducts, and related compositions. It also improves utilization ofthese compositions by keeping them in their active form. This, plus themaintenance of a system free from precipitated or other depositsimproves reliability by maintaining better control of flow rates,concentrations, and spray patterns. Other advantages will appear fromthe description below.

Various NH-containing compositions, in their pure and typical commercialforms, will generate the amidozine radical when subjected to elevatedtemperatures, either in aqueous solution or dry form. Among theprominent NH-containing compositions of concern in the present inventionare those selected from the group consisting of ammonia, urea, ureaprecursors, urea hydrolysis products, products of reaction of urea withitself or other compositions, related compositions, and mixtures ofthese. Among the hydrolysis products are ammonia, carbamates such asammonium carbamate, ammonium carbonate, ammonium bicarbonate and otherammonia salts, particularly those of organic acids such as citric andformic, various urea complexes and half ammonia salts. The exact form ofsome of these compounds is not known because the techniques employed toanalyze them can affect their makeup.

Viewed from another perspective, the invention provides a greater degreeof reliability to processes and apparatus which employ or are used inconnection with aqueous solutions of the type described. These solutionshave been found to provide an unexpectedly severe stress on the abilityto use makeup or dilution water with any significant hardness. This isgraphically shown in FIG. 1 which presents the data of Example 1 at 450ppm total hardness and also data at 45, 900, 1350 and 2250 ppm.

The presence of these NH-containing compositions in solution in theamounts effective for their usual commercial functionalities, createssignificant hardness-related problems. These problems can go unnoticedwhen water hardness values are not excessive, and arise with suchsuddenness that their cause may not be properly attributed to hardnessbut blamed instead on the purity of chemicals supplied. The problemsposed by less-than-consistent chemical quality can also stress thesystem and result in scale buildup or other problems due to hardnessprecipitation and/or collection.

In view of the discovery that water hardness problems are exacerbated bythe presence of urea and other NH-containing compositions, the inventioncan achieve its major objectives by incorporating in these solutions ahardness-suppressing composition comprising at least one member selectedfrom the group consisting of polymers, phosphonates, chelants,phosphates and mixtures of any two or more of these, in an amounteffective to suppress hardness. Thus, single members of this group canbe employed where effective, or two or more members of a single groupcan be employed, as well as mixtures of members from different groups.

Among the various hardness-suppressing compositions are: one or morepolymers; combinations of one or more polymers and one or morephosphonates; combinations of one or more polymers, one or morephosphonates, and one or more chelants; combinations of one or morepolymers, one or more phosphonates, one or more chelants and one or morephosphates; combinations of one or more polymers, one or morephosphonates, and one or more phosphates; combinations of one or morepolymers and one or more chelants; combinations of one or more polymersand one or more phosphates; combinations of one or more polymers, one ormore chelants and one or more phosphates; one or more phosphonates;combinations of one or more phosphonates and one or more chelants;combinations of one or more phosphonates and one or more phosphates; andcombinations of one or more phosphonates, one or more chelants, and oneor more phosphates. In addition, certain phosphates, alone or incombination, are effective and can be employed for scale conditioningand control as well as corrosion control. Similarly, chelants can, aloneor in various combinations, have limited effectiveness.

Typically, the pH of the aqueous solution is above 5, and generally iswithin the range of from 7 to 11, e.g., 8 to 10.

Polymers

Any water-soluble polymer which is effective to suppress hardness can beemployed. A wide variety of polymers is commercially available. Amongthese are water-soluble acrylic polymers. Exemplary of these arepolymers and copolymers (including those which are substituted orderivatized) of acrylic acid, methacrylic acid, ethyl acrylic acid,acrylamide, esters of acrylic monomers, and maleic acid or its anhydrideas described, for example in U.S. Pat. Nos. 3,890,228, 4,680,124,4,744,949, 4,752,443, 4,756,881, 4,818,506, 4,834,955, 4,904,413,4,919,821, 4,923,634, and 4,959,156 and Canadian patent 1,117,395. Thedisclosures of each of these and the references cited therein areincorporated herein by reference in their entireties. The preferredwater-soluble polymers will have molecular weights within the range offrom 500 to 300,000, typically from 1,000 to 50,000, e.g., 2,000 to25,000, as measured by gel permeation chromatography in water.

Among the effective polyacrylates (acrylic polymers) are those includingrecurring groups represented by the following general formula: ##STR1##wherein: each R independently is hydrogen or lower alkyl (e.g., 1 to 4carbons) and each Y independently is hydroxyl (OH), oxymetalic (OM),oxyorgano (OR), oxyammonium (ONH₄), amino (NR₂), amino carbosulfonic(NHRSO₃ M), amino carbosulfonic ester (NHRSO₃ R) and the like. In theseformulae: M is H or a metal, particularly one selected from the groupconsisting of alkali metals (e.g., Na, K, Li), alkaline earth metals(e.g., Ca, Mg), transition metals (e.g., Zn, Cu, Ni) and mixtures ofthese; and each R independently is hydrogen or an aliphatic, aromatic,or carbocyclic group and can be saturated or unsaturated, and eithersubstituted or unsubstituted with alkoxy, keto, carboxyamide,polycarboxy, polyhydroxy, carboxylate ester, amino, phosphonic acid,phosphate ester, sulfonic acid, sulfonate salt or ester or othercompatible subsistent.

The expressions "acrylic polymer" and "polyacrylate" as used hereininclude homopolymers, and copolymers, including terpolymers, whichcomprise one or more of the monomeric residues defined by formula I asrecurring units. Other monomers including maleic or itaconic acids andtheir derivatives and precursors, vinyl acetate (which can be hydrolizedafter polymerization to polyvinyl alcohol), lower alkyl vinyl ethers,styrene, styrene anhydride, other vinyl monomers (e.g.,3-allyloxy-2-hydroxypropane sulfonic acid), derivatized starches, andthe like can also be employed.

By way of further example, residues of maleic acid or itaconic acid ortheir anhydrides can be employed to prepare effective homopolymers orcopolymers including the residues of other monomers. They can becopolymers of two, three or more different monomers. Block copolymers aswell as random copolymers can be employed. These homo or copolymers willtypically include one or more of the following as recurring units:##STR2## wherein each Y independently is as defined above and each Zindependently is: hydrogen, lower alkyl (e.g., 1 to 4 carbons), hydroxyl(OH), oxymetalic (OM), oxyorgano (OR), alkylsulfonic (RSO₃ M),alkylsulfonic ester (RSO₃ R) and the like and M and R being as definedabove.

Among the oxyorgano (OR) substituents are mono and polyhydric alcoholresidues, particularly those having from 1 to 4 carbons and up to threehydroxyls. Prominent among these are the residues of methanol, ethanol,ethylene glycol, 1,3-propane diol and 1,3-butane diol, and isomericforms of these.

The presence of amino (NR₂) substituents on the monomers results inacrylamide monomers and polymers. Among these are the following N-, N,N-and mixed acrylamides: methyl, ethyl, propyl, i-propyl, butyl, i-butyl,t-butyl, pentyl, hexyl, carboxy pentyl, methoxy propyl, tris(hydroxymethyl) methyl, (1,2-dicarboxy) ethyl, (1,2-dihydroxy) ethyl,(2,3-dihydroxy) propyl, (2-methyl-1,2-dihydroxy) propyl,2-(2,4,4-trimethyl pentyl), 2-(2-methyl-4-oxopentyl), and likesubstituents and their isomers.

Also effective are various sulfonated amides of the above formulae wherethe amino carbosulfonic groups (NHRSO₃ R) are represented by thefollowing: 4-aminobenzene sulfonic acid, aminomethane sulfonic acid,2-aminoethane sulfonic acid, 3-aminobenzene sulfonic acid,2-amino-2-methyl propyl sulfonic acid (N(H)C(CH₃)₂ CH₂ SO₃ H) (AMPS),1-amino-2-hydroxy-3-propane sulfonic acid, and 2,3-hydroxy propylamine.

The amino carbosulfonic ester groups (NHRS₃ R) of the formula arerepresented by the alkyl, aralkyl, aryl, and carbocyclic esters of theabove amino carbosulfonic groups.

Specific polyacrylate-based polymers include but are not limited toacrylic acid/acrylamide, acrylic acid/vinyl acetate, acrylicacid/acrylate ester, acrylic acid/maleic acid/acrylamidomethylpropanesulfonic acid, and acrylic acid/acrylamido methyl-propane sulfonic acidpolymers, and combinations thereof.

Among the useful commercially-available low molecular weight polymersare the following:

    ______________________________________                                        Polyacrylates                                                                 Goodrite K-752                                                                             polyacrylate made in isopropanol from                                         Goodrich, molecular weight (MW) 2,000                            Acrysol LMW-20X                                                                            polyacrylate from Rohm & Haas,                                                MW 2,000                                                         Acrysol LMW-45X                                                                            polyacrylate from Rohm & Haas,                                                MW 4,500                                                         Nalco 1340   polyacrylic acid, MW 6,000, from Nalco                           Polymethacrylates                                                             Tamol 850    Na polymethacrylate, MW 12,000,                                               Rohm & Haas                                                      Tamol 960    Na polymethacrylate, MW 4,200,                                                Rohm & Haas                                                      Sulfonated Polymers                                                           Versa TL-4   Sulfonated styrene anhydride from                                             National Starch                                                  AMPS         2-acrylamido-2-methylpropane sulfonic                                         acid, available as monomer from                                               Lubrizol, also available from Calgon as a                                     copolymer of acrylic acid and AMPS                               Maleic Anhydride Polymers                                                     Belclene 200 polymaleate from Ciba Geigy                                      Belclene 283 polymaleic anhydride terpolymer with                                          ethyl acrylate and vinylacetate,                                              MW 1,500, from Ciba Geigy                                        Polyacrylate-Acrylamide-Methacrylate Co- or Ter-polymers                      Goodrite KXP-70                                                                            acrylate/methacrylate/t-butylacrylamide                                       terpolymer, 60/20/20 ratio, MW 8,000,                                         from Goodrich                                                    TRANSPORT PLUS                                                                             polyacrylate-acrylamide copolymer,                                            MW 25,000, from Nalco                                            Polyacrylates Modified with Hydroxy Groups                                    Gelvatol 40/20                                                                             copolymer of hydrolyzed vinylacetate and                                      vinylalcohol, from Monsanto                                      Betz 2020    acrylic acid, hyroxypropylacrylate                                            copolymer, from Betz                                             ______________________________________                                    

These polymers may be prepared using conventional polymerizationtechniques. Many, as indicated above, are commercially available. Otherscan be prepared following the techniques described in theabove-identified references.

Phosphonates

The term "phosphonate" includes phosphonic acids, including allpolyphosphonic acids and salts and esters of these acids, which arewater soluble and effective to suppress hardness in solutions of thetype disclosed herein. As with the polymers, there is a wide variety ofphosphonates which are commercially available and will be effective forthe purposes of the present invention. See U.S. Pat. Nos. 4,303,568 and4,923,634 which list a number of representative phosphonates. Thedisclosures of these patents are incorporated herein by reference.

The organophosphonic acid compounds are those having a carbon tophosphorous bond, i.e., ##STR3##

These compounds can be organized into several groups including the acid,ester and salt forms of the following: organo monophosphonates, organodiphosphonates, amino monophosphonates, and amino polyphosphonates.##STR4## wherein R is lower alkyl having from about 1 to 6 carbon atoms,(e.g., methyl, ethyl, butyl, propyl, isopropyl, pentyl, isopentyl andhexyl); substituted lower alkyl of from 1 to 6 carbon atoms (e.g.,hydroxyl and amino-substituted alkyls); a mononuclear aromatic (aryl)radical (e.g., phenyl, benzyl, etc.) or a substituted mononucleararomatic compound (e.g., hydroxyl, carboxy, amino, lower alkylsubstituted aromatic such as benzyl phosphonic acid); and M is awater-soluble cation, e.g., sodium potassium, ammonium, lithium, etc. orhydrogen.

Specific examples of compounds which are encompassed by this formulainclude: ##STR5## wherein each R^(iv) independently is hydrogen, acarboxyl (CO₂ H) group or a phosphono (PO₃ H₂) group. Among this groupis 2-phosphono-1,2,4-tricarboxybutane (PBTC).

Organo Diphosphonates ##STR6## wherein m is an integer (e.g., from 1 to12); each R^(v) independently is hydrogen or an alkyl group (e.g.,having 1 to 6 carbons); and each R^(vi) independently is hydrogen,hydroxyl or an alkyl group (e.g., from 1 to 6 carbons).

Among the preferred organophosphonic acid compounds for use in thecomposition of this invention are hydroxy alkylidene diphosphonic acidscorresponding to formula (VI) above such as those disclosed in U.S. Pat.Nos. 3,214,454, 3,297,578, and 4,923,634, the disclosures of which areincorporated herein by reference. Also suitable is an alkylenediphosphonic acid corresponding to formula (VI) such as those disclosedin U.S. Pat. No. 3,303,139, the entire disclosure of which is alsoincorporated herein by reference.

Among the suitable organo diphosphonates are:

methylene diphosphonic acid

CH₂ (PO₃ H₂)₂

ethylidene diphosphonic acid

(CH₃)CH(PO₃ H₂)₂

isopropylidene diphosphonic acid

(CH₃)₂ C(PO₃ H₂)₂

1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP)

H₂ O₃ P--C(OH) (CH₃)--PO₃ H₂

hexamethylene diphosphonic acid

H₂ O₃ P--CH₂ (CH₂)₄ CH₂ --PO₃ H₂

trimethylene diphosphonic acid

H₂ O₃ P--(CH₂)₃ --PO₃ H₂

decamethylene diphosphonic acid

H₂ 0₃ P--(CH₂)₁₀ --PO₃ H₂

1-hydroxy propylidene diphosphonic acid

H₂ O₃ PC(OH)CH₂ (CH₃)PO₃ H₂

1,6-dihydroxy-1,6-dimethyl hexamethylene diphosphonic acid

H₂ O₃ PC(CH₃)(OH)(CH₂)₄ C(CH₃)(OH)PO₃ H₂

dihydroxy diethyl ethylene diphosphonic acid

H₂ O₃ PC(OH)(C₂ H₅)C(OH)(C₂ H₅)PO₃ H₂ ##STR7## Amino Phosphonates##STR8## wherein R^(vii) is a lower alkylene having from about one toabout four carbon atoms, or an amine or hydroxy substituted loweralkylene; R^(viii) is [R^(vii) -PO₃ M₂ ], H, OH, amino, substitutedamino, an alkyl having from 1 to 6 carbon atoms, a substituted alkyl offrom 1 to 6 carbon atoms (e.g., OH, NH₂ substituted), a mononucleararomatic radical or a substituted mononuclear aromatic radical (e.g.,OH, NH₂ substituted); R^(ix) is R^(viii) or the group represented by theformula ##STR9## wherein R^(x) and R^(xi) are each hydrogen, lower alkylof from about 1 to 6 carbon atoms, a substituted lower alkyl (e.g., OH,NH₂ substituted), hydrogen, hydroxyl, amino group, substituted aminogroup, a mononuclear aromatic radical, or a substituted mononucleararomatic radical (e.g., OH and amine substituted); R^(xii) is R^(x),R^(xi), or the group R^(vii) -PO₃ M₂ (R^(vii) is as defined above);

n is an integer of from 1 through about 15; y is an integer of fromabout 1 through about 14; and M is as earlier defined.

Among these is the following: ##STR10## Amino Polyphosphonates ##STR11##wherein R^(xiii) is hydrogen, hydroxyl, or an alkyl (e.g., 1 to 10carbons) group, aryl (e.g., phenyl) group or aralkyl (e.g., benzyl)group.

Among the amino phosphonic acids of formula (VIII) are ##STR12## whereinR^(xiv) is hydrogen, alkyl, aryl, aralkyl, hydroxyl, hydroxy alkyl((CH₂)_(n) OH) or a phosphono alkyl ((CH₂)_(n) PO₃ H₂) group; and n isan integer, typically from 1 to 4.

Among this group are:

amino tri(methylene phosphonic acid) (AMP)

N(CH₂ PO₃ H₂)₃

imino-di(methylene phosphonic acid)

NH(CH₂ PO₃ H₂)₂

n-butyl-amino-di(methyl phosphonic acid)

C₄ H₉ N(CH₂ PO₃ H₂)₂

decyl-amino-di(methyl phosphonic acid)

C₁₀ H₂₁ N(CH₂ PO₃ H₂)₂

trisodium-pentadecyl-amino-di(methyl phosphonate)

C₁₅ H₃₁ N(CH₂ PO₃ HNa)(CH₂ PO₃ Na₂)

n-butyl-amino-di(ethyl phosphonic acid)

C₄ H₉ N(CH₂ PO₃ H₂)₂

tetrasodium-n-butyl-amino-di(methyl phosphonate)

C₄ H₉ N(CH₂ CH₂ PO₃ Na₂)₂

triammonium tetradecyl-amino-di (methyl phosphonate)

C₁₄ H₂₉ N(CH₂ PO₃ (NH₄)₂)CH₂ --PO₃ HNH₄

phenyl amino di(methyl phosphonic acid)

C₆ H₅ N(CH₂ PO₃ H₂)₂

4-hydroxy phenyl amino di(methyl phosphonic acid)

HOC₆ H₄ N(CH₂ PO₃ H₂)₂

phenyl propyl amino di(methyl phosphonic acid)

C₆ H₅ (CH₂)₃ N(CH₂ PO₃ H₂)₂

tetrasodium phenyl ethyl amino di(methyl phosphonic acid)

C₆ H₅ (CH₂)₂ N(CH₂ PO₃ Na₂)₂

n-hexyl amine di(methyl phosphonic acid)

C₆ H₁₃ N(CH₂ PO₃ H₂)₂

ethanol amino di(methyl phosphonic acid)

HO(CH₂)₂ N(CH₂ PO₃ H₂)₂

n-hexyl-amino (isopropylidene phosphonic acid) methyl-phosphonic acid

C₆ H₁₃ N(C(CH₃)₂ PO₃ N₂)--(CH₂ PO₃ H₂)

trihydroxy methyl, methyl amino di (methyl phosphonic acid)

(HOCH₂)₃ CN(CH₂ PO₃ H₂)₂ ##STR13## wherein R^(xv) is an alkyl grouphaving 1 to 10 carbons, (e.g., 2 to 6); and n is an integer, (e.g., from0 to 4 carbons, typically 0 to 1). Prominent among these compounds are:

hexamethylene diamine tetra(methylene phosphonic acid) (HMDTP)

(H₂ O₃ PCH₂)₂ N(CH₂)₆ N(CH₂ PO₃ H₂)₂

ethylene diamine tetra(methylene phosphonic acid (EDTP)

(H₂ O₃ PCH₂)₂ N(CH₂)₂ N(CH₂ PO₃ H₂)₂

trimethylene diamine tetra(methyl phosphonic acid)

(H₂ O₃ PCH₂)₂ N(CH₂)₃ N(CH₂ PO₃ H₂)₂

hepta methylene diamine tetra(methyl phosphonic acid)

(H₂ O₃ PCH₂)₂ N(CH₂)₇ (CH₂ PO₃ H₂)₂

decamethylene diamine tetra(methyl phosphonic acid)

(H₂ O₃ PCH₂)₂ N(CH₂)₁₀ N(CH₂ PO₃ H₂)₂

tetradecamethylene diamine tetra(methyl phosphonic acid)

(H₂ O₃ PCH₂)₂ N(CH₂)₁₄ N(CH₂ PO₃ H₂)₂

ethylene diamine tri(methyl phosphonic acid)

(H₂ O₃ PCH₂)₂ N(CH₂)₂ NH(CH₂ PO₃ H₂)

ethylene diamine di(methyl phosphonic acid)

(H₂ O₃ PCH₂)NH(CH₂)₂ NH(CH₂ PO₃ H₂)

chloroethylene amine di(methyl phosphonic acid)

ClCH₂ CH₂ N(CH₂ PO₃ H₂)₂ ##STR14## wherein m and n are as defined above.Among this group are diethylene triamine penta(methylene phosphonicacid)

(H₂ O₃ PCH₂)₂ N(CH₂)₂ N(CH₂ PO₃ H₂)--(CH₂)₂ N(CH₂ PO₃ H₂)₂

Among other phosphonates are

trietylene tetra amine hexa(methyl phosphonic acid)

(H₂ O₃ PCH₂)₂ N(CH₂ PO₃ H₂) (CH₂)₂ N--(CH₂ PO₃ H₂) (CH₂)₂ N(CH₂ PO₃ H₂)₂

monoethanol diethylene triamine tri(methyl phosphonic acid)

HOCH₂ CH₂ N(CH₂ PO₃ H₂) (CH₂)₂ NH--(CH₂)₂ N(CH₂ PO₃ H₂)₂

The water-soluble salts of these acids include the alkali metal,ammonium, amine, lower alkanolamine salts, and like salts. Among thesuitable esters are the lower alkyl (e.g., methyl and ethyl) esters.Mixtures of the organophosphonic acid compounds described above are alsocontemplated.

Phosphates

Phosphates can be employed alone or with other hardness-suppressingcompositions in amounts effective to suppress hardness. Among thesuitable inorganic phosphates are the acid forms of inorganic phosphateand any of their metal, ammonium or amine salts. Representative of theinorganic phosphates (ortho and condensed) are those chosen from thegroup: orthophosphates, pyrophosphates, tripolyphosphates,hexametaphosphates, and higher molecular weight polyphosphate oligomers.Also effective are organo phosphates such as phosphate esters,especially those of a type represented by the following structures:##STR15## where R^(xvi) =(CH₂ CH₂ O)_(q) PO₃ H₂, and q is an integer,e.g., 1 to 2; ##STR16## where q is as defined above.

Any of these phosphates may be used alone or in combination.Orthophosphates and condensed (polyphosphates) are preferred.Preferably, a combination of at least one polyphosphate and one of theother phosphates will be utilized.

Chelants

The invention can take advantage of conventional chelants, alone or incombination with other hardness-suppressing compositions, in amountseffective to suppress hardness. Among these are ethylene diaminetetracetic acid (EDTA), nitrilotriacetic acid (NTA), N-hydroxy ethylethylene diamine tetracetic acid, hydroxyethylene diamine triacetic acid(HEDTA), citric acid, diethylenetriamine pentacetic acid, gluconic acid,tartaric acid, glucoheptonic acid, and the water-soluble salts of these.

Combinations of Hardness Suppression Compositions

The hardness-suppressing composition is employed at a level effective tosuppress hardness. Preferably, a polymer and a phosphonate are employedin combination at a weight ratio of within the range of from 1:25 to25:1, but the preferred ratio is 4-6:1. Ratios outside these ranges canbe employed so long as at least a minimum effective amount of eachcomponent in the combination is employed. When a concentrate is preparedfor final dilution, it is desired to employ enough of thehardness-suppressing composition to be effective in the concentrate andall contemplated degrees of dilution.

It is an advantage of the present invention that makeup water of extremehardness (for example, blow-down water from boilers and cooling towers)can be employed. Typical levels of usage for the hardness-suppressingcomposition in urea solutions is usually less than 1% of the solution,more typically less than 0.5%. When effective pairs ofhardness-suppressing compositions cooperate in providing the beststability, the usage levels can be kept to the low end of these ranges,e.g., 1 part of the composition for from about 2,000 to 5,000 parts ofhardness, including particulate matter such as silt, clay andprecipitates. Typical levels fall within the range of 1 part of thecomposition for each 20 to 200 parts of hardness. These hardness valuesare expressed as calcium carbonate.

In the preferred embodiments, surfactants are also employed to aid inmaintaining dispersion of solution components and pH modifiers such asmonoethanolamine are employed in amounts effective to achieve long-termstability, especially when subjected to temperature extremes of fromfreezing to 120° F. One preferred hardness-suppressing formulationcomprises 83% dionized water, 2.5% of a 60% solution of HEDP, 10% of a63% active solution of polyacrylic acid (prepared using organicperoxide/isopropanol catalyst) (approximate MW 2000), 1% Igepal CO-730nonionic surfactant (nonyl phenol ethoxylate), 1% aroma enhancer, 1.5%monoethanolamine, and 1% Dowfax 3B2 anionic sulfonate surfactant(alkylated diphenyl oxide disulfonates). Among other specificembodiments are: using PBTC in place of the HEDP; using AMP in place ofthe polyacrylic acid; and, using a low molecular weight polyacrylic acidmade in aqueous media with bisulfite/persulfate catalyst and PBTC. Ineach of these specific embodiments, the hardness-suppressing formulationis employed at a level of from about 0.1 to 5% (e.g., 0.5%) in asolution containing about 50% urea.

Fertilizers

The invention has an exceptional utility in the field of agriculture.The application of nitrogenous materials to sources of vegetation (soilor other substrate either including or being prepared for viablevegetation) for fertilization can be improved. According to thisprocess, an aqueous solution is prepared comprising at least onenitrogenous fertilizer and a hardness-suppressing composition inaccordance with the invention. This solution is then applied in a mannereffective to supply nitrogen to vegetation. These nitrogenousfertilizers will comprise at least one member selected from the group ofNH-containing compositions identified above or an equivalent material.

Application of the solution to soil or other substrate is preferably byspraying, and this is also the manner which is most problematic in termsof conduit and nozzle restriction and/or blockage due to the increasedproblems associated with water hardness due to the nitrogenousfertilizer materials. The hardness-suppressing component will beemployed in an amount effective to moderate the tendency forprecipitation of hardness factors. These fertilizer solutions, either inconcentrate form or as finally diluted are often stored in sheds orbarns which subject them to extremes of temperature. Preferably, thefertilizer solutions should be stable throughout the temperature rangeof from -40° F. to 160° F. The solutions desirably have freeze/thawstability (e.g., at least three cycles of freezing and thawing) andsurvive long-term storage (e.g., at least one month).

It is an advantage of this aspect of the invention that soil nutrients,otherwise considered to be water hardness factors and applied in aqueoussolution only with some problems, can now be added up to the limits setin accordance with this invention. For example, iron, calcium,magnesium, boron, zinc, molybdenum, manganese and copper can be added tothe fertilizer solution in amounts effective for the nutrition ofvegetation, for example individually from 10 ppm to 5%, and incollective amounts of from 0.1 to 5%, the amounts depending on thecomposition of any carrier.

It is another advantage of this aspect of the invention that nutrientsolutions for hydroponic growing can be prepared and handled with greatfacility.

NO_(x) Reduction

In another of its more specific aspects, an improvement is provided inthe known process for reducing the concentration of nitrogen oxides in agas stream.

As set forth in the references identified above and those additionalones cited in the parent application, the known processes involvepreparing an aqueous solution of NO_(x) -reducing agent, transportingthe solution to the gas stream through conduits effective for thispurpose, and introducing the solution into the gas stream at aconcentration and at a rate effective to reduce NO_(x) under theconditions prevailing in the gas stream. This type of process can bepracticed with or without a catalyst, by selective gas-phase reactions.Without a catalyst the process is selective, non-catalytic reduction(SNCR), and with a catalyst it is selective catalytic reduction (SCR).

These processes are improved by incorporating in the solution ahardness-suppressing composition of the type identified above in anamount effective to increase the stability of hardness factors, andthereby moderate the tendency for precipitation of hardness factors suchas in the conduits, nozzles, or storage vessels.

Effluents in need of treatment are produced by a variety of sourcesincluding large utility boilers, circulating fluidized bed boilers, andgas turbines. It will be understood, though, that although written interms of the reduction of nitrogen oxides in the effluent from thecombustion of a carbonaceous fuel, the invention is applicable in anyhigh temperature environment having nitrogen oxides which are desired tobe reduced. By "high temperature environment" is meant an environmentwherein the temperature is sufficient to adversely affect the ability toeffectively introduce a nitrogen oxides-reducing composition therein.Such temperatures will typically be greater than about 500° F. and canbe as high as about 1900° F., even 2100° F. and higher.

In most NO_(x) -reducing processes, the treatment composition isintroduced into the effluent by an injector which generally comprises aconduit, sometimes fitted at the tip with a nozzle, extending into theeffluent. In some cases a portion of effluent or other gas is employedto help atomize and disperse the treatment composition. The spray orinjection pattern is defined with precision, often with the aid ofcomputer to assure good distribution and reaction. The present inventionhelps assure operation according to the defined pattern by minimizinginjector fouling.

Apparatus of varying degrees of sophistication are known for introducingNO_(x) -reducing compositions into a high temperature environment. Somecomprise coaxial, multi-tubular structures, such as those disclosed byBurton in U.S. Pat. No. 4,842,834, and by DeVita in U.S. Pat. No.4,985,218, the disclosures of each of which are incorporated herein byreference.

Among the problems of injecting NO_(x) -reducing compositions at hightemperatures is that flow anomalies can arise due to a number ofproblems associated with hardness factors. These problems becomecritical because of flow stoppages, flow rate alteration, spray patternalteration, droplet size deviation, liquid impingement on equipmentsurfaces, reaction temperature window modification, and the like. Thus,high temperature operations are matters of considerable delicacy. Flowproblems which might be minor under ambient conditions are exaggeratedin severity at high temperature because reduced flow rates and partialblockages can soon result in total blockage and even system failure.

These problems and others make the control of SNCR and SCR systemsdifficult. These processes require individual molecules of NO_(x)-reducing agents (or, more precisely, free radical components of them,e.g., the amidozine radical) to react with individual NO_(x)molecules--both the reducing agent and the NO_(x) being present at verylow (ppm) concentrations. The control system should be adequate to varyconcentration and composition of reducing agent at boiler load and otherfactors cause the NO_(x) concentration and effluent temperature profileto vary. Any mismatch will cause final effluent NO_(x) or otherpollutant levels to be excessive.

The presence of uncontrolled hardness in urea and other NH-containingcompositions can cause a variety of adverse effects. It can result inscaling with consequent flow restriction and blockage of conduits andnozzles. Large pieces of scale can flake off and cause instantaneousblockage. At high temperature, flow restriction will not only destroythe desired spray pattern, but can cause further flow problems as theheat increases the viscosity of the fluid, causes localized boiling, orchanges the chemical composition of the NO_(x) -reducing composition. Inaddition, the hardness factors can combine with the active chemicals toform insoluble salts and, cause precipitation of the active chemicalseven at ambient temperatures.

Storage vessels and conduits will thus accumulate these precipitates,and volumetric control systems will deliver less active chemical to thedesired zone. Precipitation can also cause viscosity changes which alterspray patterns from design. It is an advantage of this invention thathardness factors in NH-containing-solutions can now be controlledreliably due to the identification of the NH-containing compositions,themselves, as parameters which affect hardness instability.

The present invention is directed to reducing the effect of hardnesscomponents, and their adverse effects at all temperatures. Thecompositions of this invention can be employed with compositions havingother additives such as surfactants, pH modifiers, odorants, and others.

In situations where an aqueous NO_(x) -reducing composition has aviscosity sufficiently high to substantially interfere with its flow atthe high temperature conditions which exist at an injector tip and/orhas a tendency to coke at such high temperature conditions, it isimportant to insure adequate introduction of the component into theeffluent by providing a viscosity/coking time modifier as described inU.S. application Ser. No. 07/576,424, identified above. The disclosureof the above-identified parent applications are hereby incorporated intheir entireties for their disclosure of these and other additives whichare useful in commercial treatment compositions.

The NO_(x) -reducing composition improved by the invention willtypically be prepared and shipped as a concentrate which is diluted foruse. Typically, these concentrates will contain 25 to 65% urea and 0.05to 1.0% of an effective hardness-suppressing composition, morepreferably from 40 to 55% urea, e.g., 50%, and from 0.1 to 0.75%, e.g.,0.5%, of the hardness-suppressing composition. This concentrate isdiluted as required to achieve a urea concentration effective under theconditions. Typically, dilution to concentrations of from 5 to 25% ureaare effective. Lower concentrations (e.g., 1 to 5%) may be desired.

It is an advantage of the invention that the dilution water can haveunusually high hardness levels and still meet the objectives. Forexample, there is a broad range of waste water streams produced byindustrial plants which are regulated as to methods and quantity ofdisposal. By selection of the appropriate hardness-suppressingcomposition within the guidelines disclosed it is possible to employthese waste water streams as dilution water (to bring the concentrationof NH-containing composition to the desired level) and therebyfacilitate the non-polluting disposal of these streams.

Exemplary of the waste water streams which can be employed as dilutionwater are blow down water from a source employing recirculating coolingwater where hardness tends to build up unless periodically "blown down"and replaced in part with fresh makeup water (e.g., cooling towers andboilers); brines from various sources such as reverse osmosis; wastewater streams from various cleaning processes, including air heater washwater; and the like. By analysis of the particular waste water stream,it is then possible to employ the correct combination ofhardness-suppressing treatment agents in the NH-containing concentrateitself or as a separate additive package to be added as part of thefinal aqueous solution. An additive of this type is preferably dispersedin the waste water prior to addition to the concentrate or into theNH-containing concentrate prior to dilution.

For boiler water used as dilution water, the hardness-suppressingcomposition can contain: chelant(s); polymer(s); a combination ofchelant(s) and polymer(s); a combination of phosphonate(s) andpolymer(s); a combination of chelant(s) , phosphonate(s) and polymer(s);and combinations of any of these with a phosphate such astripolyphosphate which is employed for scale control. For cooling towerblowdown water used as dilution water, the hardness-suppressingcomposition can contain these same combinations but chelants are per senot as important and phosphates (including esters) may be employed forcorrosion control. For waste water treatment, cationic and anionicpolymeric dispersants can be important.

The mechanisms by which the scale control agents work in urea solutionsas well as other kinds of water solutions are by either thresholdactivity, chelation or both effects in combination. Threshold mechanismsare the primary and preferred method because of cost and efficiency.Chelation may occur at least in part in the soft water (e.g., 45 ppm)testing.

Threshold inhibition refers to the phenomenon where inhibitors preventprecipitation of mineral salts when added in amounts which are less thanthe amount of the scaling ion. Threshold inhibitors are typically viewedas acting as particle dispersion by steric stabilization andelectrostatic repulsion, and retarding crystal growth by adsorbing ontoand blocking active growth sites. Chelation is the binding between theinhibitor and the metal ion (e.g., Ca, Mg, Fe ions) at two or moresites. The complexation of metal ion with chelant scale inhibitorresults in dissolution of the metal ion. Chelation requires a 1:1 moleratio of chelating agent (e.g., citric acid, EDTA, etc.) to metal ion,and is therefore stoichiometric. The ratio of threshold inhibitor toscaling ions is generally much smaller and is substoichiometric.

Typically, threshold inhibitors (e.g., polymers, phosphonates) areapplied at a dosage ratio of 1:2-5000 ppm active inhibitor/ppm totalhardness or particulate matters such as silt, clay or precipitate.

The NO_(x) -reducing agents based on carbamates and urea hydrolysates,such as those described in U.S. Pat. No. 4,997,631 to Hofmann et al, andU.S. patent application Ser. No. 07/561,154, filed Aug. 1, 1990 in thenames of von Harpe et al, particularly stress the ability of thesolution to retain hardness factors in soluble form. This patent and theapplication are incorporated herein by reference. Accordingly, thepresent invention is of particular advantage when dealing with materialsof this type or its components such as ammonium or calcium carbonates,bicarbonates and carbamates.

EXAMPLES

The following examples further illustrate and explain the invention.Unless otherwise indicated, all parts and percentages are by weight, andhardness values for total hardness, calcium and magnesium are alwaysexpressed as calcium carbonate.

A series of test solutions was prepared having the following commoncharacteristics:

    ______________________________________                                        Total hardness (as CaCO.sub.3)                                                                   450 ppm                                                    Calcium/Magnesium (ppm ratio)                                                                    2/1                                                        pH                 9.3 to 9.7                                                 NaHCO.sub.3 Alkalinity (as CaCO.sub.3)                                                           300 ppm                                                    Silica             0, 60 or 150 ppm                                           Minimum Conductivity (approx)                                                                    5000 micromhos                                             Urea Solution Concentrations                                                                     0% as Blank                                                                   also at 5%, 15%, 20%, and                                                     25% (15% is the most                                                          typical)                                                   Hardness-Suppressing Composition                                                                 0% in 15% Urea                                             Concentration      Solution as Control                                        ______________________________________                                    

Tests were also performed varying the total hardness to levels of 45 ppm(Example 6) and 2250 ppm (Example 5). The dosage in terms of the ratioof ppm active hardness suppressing compositions/ppm total harness wastypically <50%, and was as low as 1%. An adjustment of conductivity to aminimum of 5000 micromhos was made by adding a 2:1 NaCl/Na₂ SO₄ solutionfor the blank and 45 ppm total hardness water. Various chemicalcompositions were added for scale inhibition (a form of hardnesssuppression) according to a modified National Association of CorrosionEngineers (NACE) Standard Test Method TM-0374-90 (Item No. 53023). TheNACE procedure is recommended by the Technical Practices Committee forthe testing of calcium carbonate and calcium sulfate precipitation. Thedetails of the NACE procedure may be found in "Laboratory ScreeningTests to Determine the Ability of Scale Inhibitors to Prevent thePrecipitation of Calcium Sulfate and Calcium Carbonate from Solution,"approved in November, 1974 and revised in January, 1990.

The testing conditions were modified to simulate conditions to beexpected in storage and use of urea solutions. Typically, a higher pHwater gives a more severe scaling condition. The high range of pH in thetest condition was chosen to provide a worst-case scenario. Each testsolution was placed in an oven at 65° C. (149° F.) for three days. Eachsolution was then filtered through a 0.45 micron MILLIPORE filter, andthe supernatant (filtrate) analyzed by inductively-coupled plasmaspectroscopy (ICP) for soluble calcium, magnesium, and silica. Hardnessanalysis was performed using the well-known EDTA titration method. Thepercent inhibition was then calculated by the following equation (firstsubtracting the presence of soluble calcium in a blank from the sampleand then dividing by the initial solubility of calcium in the sampleafter subtraction of calcium initially present in the blank): ##EQU1##where C_(a) =calcium ion concentration in the sample after three days

C_(b) =calcium ion concentration in the blank after three days

C_(c) =initial calcium ion concentration in the blank.

The precipitates identified were mostly CaCO₃, and under severe hardnessconditions, could also contain Mg(OH)₂, SiO₂, nitrogen compoundsprecipitated from urea and its derivatives, and a small amount ofphosphorous, as analyzed by X-ray fluorescence spectroscopy, X-raydiffraction, and a carbon-hydrogen-nitrogen (CHN) analyzer.

"Scale" is considered by some as hard, adherent, heat-transfer hinderingand nozzle orifice blocking solids. No differentiation was made between"precipitate" and "scale". The invention encompasses hardnesssuppression in terms of both inhibition and prevention of scaleformation and dispersion and stabilization of precipitates.

Example 1

The severity of scaling in the urea solution as compared to watercontaining the same amount of impurities is shown graphically in FIG. 1.The specifics of testing for one of these hardness levels is set forthin Table 1 and 1A. The 15% urea solution exhibited virtually nosuppression (0%) of the mineral scale at the 450 ppm hardness level. Theprecipitate contained nitrogen compounds from urea and/or itsderivatives. Due to the NACE method of calculation, the 15% ureasolution (control) becomes 0% by definition and other samples arecalculated relative to it. The urea Control has a much worse scalingcondition than the Blank.

                  TABLE 1                                                         ______________________________________                                        For water containing 450 ppm hardness (`H`) and 60 ppm SiO.sub.2 :                   ppm Retained.sup.1                                                                             % Ca                                                  % Urea   `H`      Ca    Mg     SiO.sub.2                                                                          Suppression                               ______________________________________                                         0       199      78    131    65   23                                        15       162      11    153    66    0                                        ______________________________________                                         .sup.1 averaged values                                                   

                  TABLE 1A                                                        ______________________________________                                        For the water containing 450 ppm hardness and no silica:                               ppm Retained.sup.1                                                                          % Ca                                                   % Urea     `H`    Ca        Mg   Suppression                                  ______________________________________                                         0         280    143       147  46                                           15         160     11       150   0                                           ______________________________________                                         .sup.1 averaged values                                                   

Comparison of Tables 1 and 1A shows that presence of silica in anuninhibited water makes the base water less stable.

Example 2

A combination of polyacrylic acid (PAA), a polymer having a molecularweight of approximately 2,000, and 1-hydroxyethylidene-1,1-diphosphonicacid (HEDP) showed 70 to 80% suppression in 15% urea solutions ascompared to 0% suppression with the untreated solutions (Tables 2, 2A).Furthermore, the treated sample gave a precipitate which was dispersedand non-adherent, as compared to hard and sticky scale in the untreatedsample.

                  TABLE 2                                                         ______________________________________                                        For the 15% urea solution containing 450 ppm total                            hardness and no silica:                                                       ppm Suppressor.sup.2                                                                            ppm Retained.sup.1                                                                         % Ca                                           % Urea  HEDP    Polymer   `H`  Ca   Mg   Suppression                          ______________________________________                                        15      0       0         160   11  150   0                                   15       7.4    31.5      375  240  150  79                                   15      14.8    63.0      364  220  150  72                                   15      22.2    94.5      358  220  150  72                                   ______________________________________                                         .sup.2 as active                                                         

                  TABLE 2A                                                        ______________________________________                                        For water containing 450 ppm total hardness and 60 ppm silica:                %     ppm Suppressor.sup.2                                                                       ppm Retained.sup.1                                                                            % Ca                                       Urea  HEDP    Polymer  `H`  Ca   Mg   SiO.sub.2                                                                          Suppression                        ______________________________________                                        15    0       0        162   11  153  60    0                                 15    22.2    94.5     351  213  143  61   70                                 ______________________________________                                         .sup.2 as active                                                         

Example 3

The PAA polymer and HEDP phosphonate combination further showedsignificant hardness suppression in a high urea (25%) solutioncontaining 450 ppm of total hardness. Table 3 comprises the 25% ureatest solution to a control having 15% urea.

                  TABLE 3                                                         ______________________________________                                        ppm Suppressor.sup.2                                                                            ppm Retained.sup.1                                                                         % Ca                                           % Urea  HEDP    Polymer   `H`  Ca   Mg   Suppression                          ______________________________________                                        15      0       0         160   11  150   0                                   25      37.0     157.5    366  210  140  69                                   ______________________________________                                    

Example 4

The combination of PAA polymer and HEDP phosphonate exhibitedadvantageous scale control capabilities over each when used alone (Table4). For example, a combination of HEDP and PAA polymer gave 50 to 70%suppression; whereas either the HEDP or PAA when used by itself oftenshowed significantly lower suppression with values as low as 10-15%.

                  TABLE 4                                                         ______________________________________                                        ppm Suppressor    ppm Retained % Ca                                           Phosphonate                                                                             Polymer     `H`    Ca   Mg   Suppression                            ______________________________________                                        22.5 HEDP     94.5   PAA    351  213  143  70                                 3.75 HEDP     15.75  PAA    358  210  160  69                                 1.5  HEDP     6.3    PAA    328  160  160  52                                 22.5 HEDP     0             328  180  150  59                                 0             94.5   PAA    194   48  130  13                                 0             92.0   PAA    170   44  120  11                                 23.0 AMP      94.5   PAA    364  210  160  69                                 25.0 AMP      0             334   23.sup.3                                                                          140   4                                 37.4 PBTC     32.4   PAA    378  120.sup.3                                                                          160  38                                 22.4 PBTC     94.5   PAA    386   80.sup.3                                                                          140  24                                 25.0 PBTC     92.0   AMS    162   41  120  10                                 25.0 PBTC     0             276   33.sup.3                                                                          140   8                                 0             92.0   AMN    138   25  130   5                                 25.0 HMDTP    0             194   26  140   5                                 0             92.0   PMA    224   80  140  24                                 ______________________________________                                         .sup.3 Post precipitation occurred between hardness titration and calcium     ion measurement by ICP.                                                  

where:

HEDP=1-hydroxyethlidene-1,1-diphosphonic acid

AMP=amino tri(methylene phosphonic acid)

PBTC=2-phosphono-1,2,4-tricarboxybutane

HMDTP=hexamethylene diamine tetra(methylene phosphonic acid)

PAA=polyacrylic acid or polyacrylate polymer

AMS=terpolymer of acrylamide, acrylic acid and sulfomethylacrylamide

PMA=polymaleic acid or polymaleate

AMN=terpolymer of acrylic acid/methacrylic acid/t-butylacrylamide

Example 5

Under the extremely severe scaling condition of 2250 ppm total hardnessand high conductivity (>6000 micromhos), low levels of the combinationof polymer(s) and phosphonate(s) were shown to be effective in 2.5% and5% urea solutions (Table 5).

                                      TABLE 5                                     __________________________________________________________________________    ppm Suppressor       ppm Retained                                                                            % Ca                                           % Urea                                                                             HEDP                                                                              PAA PBTC                                                                              AMS `H`                                                                              Ca  Mg Suppression                                    __________________________________________________________________________    15   0   0   0    0   714                                                                              13 690                                                                               0                                             (Control)                                                                     15   22.2                                                                              94.5                                                                              0    0   844                                                                             190 600                                                                              12                                             15   0   0   60.0                                                                              222  490                                                                              38 438                                                                               2                                              5   7.4 31.5                                                                              60.0                                                                              222 1400                                                                             660 740                                                                              44                                              5   7.4 31.5                                                                              90.0                                                                              333 1646                                                                             890 820                                                                              .sup. 59.sup.4                                   2.5                                                                              3.7 15.8                                                                              0    0  1388                                                                             560 770                                                                              37                                               2.5                                                                              3.7 15.8                                                                              60.0                                                                              222 1966                                                                             1300                                                                              830                                                                              .sup. 87.sup.4                                 __________________________________________________________________________     .sup.4 no scale                                                          

Example 6

A combination of HEDP with polyacrylate polymer exhibited 98% and 100%suppression in 15% and 25% urea solution for water containing 45 ppmhardness. The test is summarized in Table 6.

                  TABLE 6                                                         ______________________________________                                        ppm Suppressor   ppm Retained                                                                             % Ca                                              % Urea HEDP     Polymer  `H`      Suppression                                 ______________________________________                                         0     0        0        44.0     98    (Blank)                               15     0        0        40.8     91    (Control)                             15      22.2     94.5    44.0     98                                          25      37.0     157.5   45.2     100                                         ______________________________________                                    

The above description is for the purpose of teaching the person ofordinary skill in the art how to practice the present invention, and itis not intended to detail all of those obvious modifications andvariations of it which will become apparent to the skilled worker uponreading the description. It is intended, however, that all such obviousmodifications and variations be included within the scope of the presentinvention which is defined by the following claims.

We claim:
 1. In a process for reducing the concentration of nitrogenoxides in a gas stream by preparing an aqueous solution of NO_(x)-reducing agent, which comprises an NH-containing composition selectedfrom the group consisting of ammonia, urea, urea precursors, ureahydrolysis products, carbamates, ammonium carbonate, ammoniumbicarbonate, cyanurates, ammonium sales of organic acids, otheramidozine-generating compositions and mixtures of these; transportingthe solution to the gas stream through conduits effective for thispurpose; and introducing the solution into the gas stream at aconcentration and at a rate effective to reduce NO_(x) by selectivegas-phase reaction under the conditions prevailing in the gas stream,the improvement comprising:incorporating in the solution a hardnesssuppressing composition selected from the group of water solublepolymers effective to suppress hardness, water soluble phosphonatescomprising a member selected from the group consisting of organophosphonates, amino phosphonates and amino polyphosphonates effective tosuppress hardness, water soluble chelants effective to suppresshardness, water soluble phosphates effectivc to suppress hardness andmixtures of these, in amounts effective to reduce the tendency forprecipitation of hardness factors in the conduits.
 2. An improvedprocess according to claim 1 wherein the aqueous solution of NO_(x)-reducing agent is prepared by diluting a concentrate comprising from 15to 60% of the NO_(x) -reducing agent and from 0.05 to 1.0% of thehardness-suppressing composition.
 3. An improved process according toclaim 1 wherein the NO_(x) -reducing agent comprises urea or at leastone of its hydrolysis products or a salt thereof.
 4. An improved processaccording to claim 1 wherein the aqueous solution comprises both awater-soluble polymer and a phosphonic acid or salt ashardness-suppressing compositions.
 5. In a process for reducing theconcentration of nitrogen oxides in a gas stream by preparing an aqueoussolution of NO_(x) -reducing agent, which comprises an NH-containingcomposition selected from the group consisting of urea precursors, ureahydrolysis products, carbamates, ammonium carbonate, ammoniumbicarbonate, cyanurates, and mixtures of these; transporting thesolution to the gas stream through conduits effective for this purpose;and introducing the solution into the gas stream at a concentration andat a rate effective to reduce NO_(x) by selective gas-phase reactionunder the conditions prevailing in the gas stream, the improvementcomprising:incorporating in the solution a hardness suppressingcomposition selected from the group of water soluble polymers effectiveto suppress hardness, water soluble phosphonates comprising a memberselected from the group consisting of organo phosphonates, aminophosphonates and amino polyphosphonates effective to suppress hardness,water soluble chelants effective to suppress hardness, water solublephosphates effective to suppress hardness and mixtures of these, inamounts effective to reduce the tendency for precipitation of hardnessfactors in the conduits.
 6. A process according to claim 5 wherein theaqueous solution comprises both a water-soluble polymer and aphosphonate as hardness-suppressing compositions and additionallycomprises a surfactant.
 7. A process according to claim 5 wherein the pHis within the range of from 7 to 11, the polymer comprises a memberselected from the group consisting of polyacrylic acid,2-acrylamido-2-methylpropane sulfonic acid, and mixtures of these, andthe phosphonate comprises a member selected from the group consisting of1-hydroxy ethylidene-1,1-diphosphonic acid and2-phosphono-1,2,4-tricarboxy butane.
 8. In a process for reducing theconcentration of nitrogen oxides in a gas stream by preparing an aqueoussolution of NO_(x) -reducing agent, which comprises an NH-containingcomposition comprising a member selected from the group consisting ofurea hydrolysis products, carbamates, ammonium carbonate, ammoniumbicarbonate, and mixtures of these; transporting the solution to the gasstream through rate effective to reduce NO_(x) by selective gas-phasereaction conduits effective for this purpose; and introducing thesolution into the gas stream at a concentration and at a under theconditions prevailing in the gas stream, the improvementcomprising:incorporating in the solution a hardness suppressingcomposition comprising a member selected from the group of water solubleacrylic polymers having molecular weights of from 500 to 50,000effective to suppress hardness, water soluble phosphonates comprising amember selected from the group consisting of organo phosphonates, aminophosphonates, amino polyphosphonates, and mixtures of these, effectiveto suppress hardness, water soluble chelants effective to suppresshardness, water soluble phosphates effective to suppress hardness andmixtures of these, in amounts effective to reduce the tendency forprecipitation of hardness factors in the conduits.
 9. A processaccording to claim 8 wherein the aqueous solution comprises a surfactantand both a water-soluble polymer and a phosphonate ashardness-suppressing compositions.
 10. A process according to claim 9wherein the pH is within the range of from 7 to 11, the polymercomprises a member selected from the group consisting of polyacrylicacid, 2-acrylamido-2-methylpropane sulfonic acid, and mixtures of these,and the phosphonate comprises a member selected from the groupconsisting of 1-hydroxy ethylidene-1,1-diphosphonic acid and2-phosphono-1,2,4-tricarboxy butane.
 11. In a process for reducing theconcentration of nitrogen oxides in a gas stream by preparing an aqueoussolution of NO_(x) -reducing agent, which comprises an NH-containingcomposition comprising a member selected from the group consisting ofurea hydrolysis products, carbamates, ammonium carbonate, ammoniumbicarbonate, and mixtures of these; transporting the solution to the gasstream through conduits effective for this purpose; and introducing thesolution into the gas stream at a concentration and at a rate effectiveto reduce NO_(x) by selective gas-phase reaction under the conditionsprevailing in the gas stream, the improvement comprising:incorporatingin the solution a hardness suppressing composition comprising watersoluble acrylic polymer having molecular weights of from 500 to 50,000effective to suppress hardness; and water soluble phosphonate comprisinga member selected from the group consisting of organo phosphonates,amino phosphonates, amino polyphosphonates effective to suppresshardness, and mixtures of these, in amounts effective to reduce thetendency for precipitation of hardness factors in the conduits.
 12. Aprocess according to claim 11 wherein which further comprises asurfactant, and the pH is within the range of from 7 to 11, the polymercomprises a member selected from the group consisting of polyacrylicacid, 2-acrylamido-2-methylpropane sulfonic acid, and mixtures of these,and the phosphonate comprises a member selected from the groupconsisting of 1-hydroxy ethylidene-1,1-diphosphonic acid and2-phosphono-1,2,4-tricarboxy butane.